1. Field of the Invention
The present invention relates to magnetic alloys, especially to iron-silicon alloys which can be rolled into a sheet.
2. Prior Art
Iron-silicon alloys, so-called silicon steel are widely used for electrical purposes, such as, core material for generators and transformers, since the iron-silicon alloys are higher in saturation magnetic induction, permeability, electrical resistivity and lower in cost, as compared with iron.
The iron-silicon alloys which contain small amounts of silicon can be easily rolled and widely used for the above mentioned electrical purposes. While it is known that in the iron-silicon alloys as the amount of silicon increases, magnetotriction and magnetic anisotropy of the alloy decrease, especially at a silicon content of 6.5 weight percent, the magnetostriction of the alloy becomes zero which results in improved magnetic characteristics as soft magnetic material that is high in permeability and electrical resistivity, and low in iron loss.
The sheet silicon steel was drastically improved when grain obtained sheet silicon steel was proposed in which orientation of crystal axis of secondary recrystallization after rolling is selected along (011) [100] directions, which results in low iron loss, high permeability along the rolling direction and high saturation magnetic induction. Further bidirectionally oriented sheet silicon sheet was proposed which exhibited superior magnetic characteristics along directions along and perpendicular to rolling.
Recently, as the increase of demands for electric power supply, transformer substations of large capacity are built in the streets area, it is the problem to reduce a noise caused by the transformer. It is known that the noise of the transformer is caused mainly by vibrations due to magnetostriction of the cores which form the transformer, and it is desirable to employ silicon rich iron-silicon alloys having small magnetostriction as core material for the transformer.
In the iron-silicon alloys, as the amount of silicon increases, hardness of the alloy increases, especially in the alloy containing not less than 4.5 weight percent silicon, malleability of the alloy is lost suddenly, and the rolling of the alloy becomes difficult which is a large bottleneck for industrial use of the iron-silicon alloys.
The reports on the rolling of iron-silicon alloy containing 6.5 weight percent silicon which has poor malleability can scarcely found, however, a report titled "Rolling of brittle metal" appeared in Journal of Japan Society for Technology of Plasticity Vol. 8, No. 77 (June 1967) pages 317-321 reports that iron-silicon alloy containing 6.5 weight percent silicon having a thickness of 10 mm could be rolled into a sheet having a thickness 0.018 mm by rolling with different material layers which were rich in malleability strongly adhered on both sides of the iron-silicon alloy. However, in this case even the iron-silicon alloy sheet was obtained experimentally by employing assistant material, it is quite difficult to obtain the iron-silicon alloy sheet of large amount stably without using special technique in industrial scale.
"Cold rolling of 6.5% Si-Fe alloys and magnetic characteristics" appeared in Journal of Japan Society of Metals Vol. 30 (1966) pages 552-558 and reported that iron-silicon alloy containing 6.5 weight percent silicon was rolled at a temperature between 600.degree. to 750.degree. C. with a rolling ratio of 70%, and after that both sides of the alloy sheets were cut out where a number of cracks were generated. Then the sheet of 1 mm thickness was rolled to the sheet having not more than 0.3 mm thickness. However, in this paper, it is reported that when rolling the iron-silicon alloy containing more than 5.0 weight percent silicon to a thickness of thinner than 1 mm, cracks were frequently generated, and it was practically difficult to make alloys sheets without generation of cracks.
It is known that the magnetostriction of the alloy is strongly affected by mechanical stress applied to the alloy by mechanical working such as bending or rolling. Then, even if silicon rich iron-silicon alloys having small magnetostriction are employed, it is difficult to decrease the magnetostriction caused by the mechanical stress due to the poor workability of the alloy, which results in unsatisfactory results in decreasing the noise of transformer.